The present invention relates to driving liquid crystal materials, and more particularly to driving such materials using low voltage techniques.
Various liquid crystal (LC) displays (LCDs), such as twisted-nematic (TN) mode LC (TNLC) displays, use a varying voltage ramp to drive the LC material to a selected gray scale level (i.e., percentage of optical rotation of polarized light). This varying voltage ramp is typically in excess of 5 volts. Since the desired result is a gray scale video rate image, a frame time of 16.7 milliseconds (ms) (i.e., 60 Hertz (Hz)) is required. Typical TNLC materials take several milliseconds to switch from one state to another, and barely respond at video rates, and therefore do not provide a response time needed to react to very short voltage pulses.
Thus, acceptable drive schemes for TNLC displays utilize full frame time variable voltage analog or analog-like systems. However, such analog or amplitude modulated (AM) systems suffer from performance issues. These performance issues include the need for high voltages (greater than 5 volts, and typically 7.5 volts) for driving the display and gray scale performance (i.e., shading steps from black to white) that is non-linear. Further, this non-linear performance curve has significant changes in voltage position and slope (i.e., change per voltage increment) over small temperature changes, and thus deleteriously affects display performance.
Also a variable voltage (VV) drive is extremely sensitive to capacitive effects such as those caused by pixels that have dielectric coatings over electrode surfaces. This typically causes an offset of performance as the LC cell is driven in one electrical polarity versus the reverse polarity. Thus a typical VV drive scheme provides an optical response that is very asymmetrical. This asymmetrical response can lead to charge buildup in the cell, causing image sticking and may further result in damage to the LC display over time. Thus a need exists to provide a display to overcome these drawbacks.